Fuel cells are electrochemical devices that process fuel and oxygen to produce electricity and heat. Fuel cells include electrodes having an anode, a cathode and an electrolyte therebetween. Fuel cells can be provided in planar and/or tubular forms. Planar fuel cells can be stacked to form piles of cells. Tubular fuel cells can be arranged in multi-tube planar arrays which can then be stacked one upon another. However, sealing between each of the planar arrays and structural supporting of the tubular fuel cells relative to one another can be problematic. Solid Oxide Fuel Cells (SOFC) operate at temperatures in the range of 900 to 1000 degrees Celsius and thus can be subject to severe thermal transients. SOFCs use hard and brittle ceramic compounds at least for the electrolyte. Such ceramic materials have poor thermal shock resistance. Although, tubular electrodes can be designed to be more resistant to thermal shock than their planar counterparts, prior art multi-tubular SOFC stacks can also be susceptible to thermal shock induced wear and reduced life, for the reasons mentioned above.
Referring to FIG. 1, a prior art multi-tube array of tubular fuel cells for a Solid Oxide Fuel Cell assembly (SOFC) is generally indicated by element number 900, wherein the array includes a plurality of tubular fuel cells 910 each having a fuel inlet 912 and a fuel outlet 914, secured within a housing 916. N. M. Sammes, Y. Du, and R. Bove, Journal of Power Sources 145, (2005), 428-434. An air passage 918 is defined between the tubes 910 for flowing air from an air inlet 920 to an air outlet 922.
Referring to FIGS. 2 and 3, in the prior art, adjacent tubular fuel cells 910 are connected to one another by respective current collectors 924 which each have a female connector 926 and male connector 928 projecting therefrom. The female connectors 926 are brazed onto an outer surface 930 of each of the tubular fuel cells 910 and the male connectors 928 are brazed onto an inner surface (not shown) of each of the tubular fuel cells 910. N. M. Sammes, Y. Du, and R. Bove, Journal of Power Sources 145, (2005), 428-434. Such an arrangement can be susceptible to thermal distortion because of the limited restraint provided between adjacent tubular fuel cells 910 by the current collectors 924.
Prior art multi-tubular SOFC stacks generally utilize individual tubular fuel cells manufactured in relatively expensive piece-part operations. Assembly of multi-tubular SOFC stacks, which utilize prior art individual fuel cells, is also complex, quite expensive and time consuming because of the multitude of parts which must be fit and attached to one another.
Material selection for SOFCs is difficult because of the high operating temperatures and thermal shock which they are subject to. However, Cordierite (2MgO.2Al2.5SiO2) is known to display very low thermal expansion over wide temperature ranges and thus demonstrates excellent thermal shock resistance. U.S. Pat. No. 3,885,977, to Lachman et al. It is also known that cordierite honeycomb structures are used in automotive applications as a structural body and catalyst carrier. U.S. Pat. No. 6,589,627, to Nakanishi et al. As noted by A. Sleight in Nature Vol. 425 (2003), pp 674-676, cordierite has a hexagonal structure, in which thermal expansion along two axes is compensated for by the opposite sign of thermal expansion along a third axis.
U.S. Pat. No. 7,037,875 discloses a support for a catalyst for controlling vehicular exhaust emissions comprising a high surface area refractory metal oxide, e.g., gamma-alumina, having a monomolecular layer of a second oxide selected from the group consisting of titanium dioxide, cerium dioxide and zirconium dioxide. The support may be converted into a vehicular exhaust emission control catalyst by depositing the support on a substrate such as cordierite. Also, surface modification of cordierite by alumina, titania, and zirconia “wash-coating” is reported by C. Agrafiotis and A. Tsetsekou in Journal of European Ceramic Society 20, (2000), 815-824, and 20, (2000), 825-834. In addition, the prior art teaches methods for processing cordierite material in a variety of forms, with a porosity of 70 percent or more. S. Izuhara, K. Kawasumi, and M. Yasuda, Ceramic Transactions 112, (2000), 553-558. T. Heinrich, W. Tappert, W. Lenhard, and J. Fricke in Journal of Sol-Gel Science and Technology 2, (1994), 921-924, report on the processing and properties of a cordierite aerogel.
Thus there is a need to provide tubular fuel cell assemblies that are resistant to thermal shock and distortion, are simple and inexpensive to manufacture and assemble and that have an extended useful life compared to that of prior art fuel cells. Prior art apparatuses, methods and systems for addressing these needs are either too expensive, too complicated, ineffective or a combination of all of these. Based on the foregoing, it is the general object of the present invention to improve upon prior art tubular fuel cell assemblies and methods and overcome the problems and drawbacks thereof.